It might have just died, these are shortlived plants, 15 years being a pretty good age. Some survive a lot longer, but usually not more than 25 years at most. However, it could also be drought which caused the trouble - if you didn't water during the drought back in the late spring/early summer, it went on for quite a while and the plant may have been going short of water, especially here in London, despite its maturity. If you can bear to keep looking at it, and you're not sure its entirely dead, leave it in place (don't prune now either) till next spring - if it puts out new shoots, fine, if it doesn't, remove it.

Sound like what happend to mine, we had to cut it down in the end and found it had gone rotten in the centre of the trunk it was 10yrs old. Good Luck with yours :o))